Recently, there has been enormous interest in growth of Group III nitride, and particularly gallium nitride (GaN) thin films, Jpn. J. Appl. Phys. Vol. 34 (1995) pp. L 797-L 799. GaN, and related (Aluminum, Indium)N alloys are being utilized for the production of efficient optoelectronic devices, e.g. light emitters and detectors spanning the spectral range of visible to deep ultra-violet (UV). In addition, the direct wide bandgap and the chemical stability of Group III nitrides are very beneficial for high-temperature and high-power operated electronic devices, e.g. hetero-junction bipolar and field effect transistors. However, the poor material quality of GaN severely limits the efficiency of such devices.
When GaN is directly grown on a sapphire substrate, the growth mode is three-dimensional due to the large lattice mismatch, the chemical dissimilarity, and the thermal expansion difference. The layer contains structural defects such as, threading dislocations, stacking faults, and point defects. These defects degrade the film's morphology and optical and electrical properties. In order to achieve high quality epitaxial growth, researchers have introduced a thin low-temperature grown AlN or GaN layer serving as a buffer layer. This layer provides nucleation sites for subsequent two-dimensional GaN growth at higher temperatures, see H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, Jpn. J. Appl. Phys. 28, L2112 (1989) and S. Nakamura, T. Mukai, M. Senoh, and N. Isawa, Jpn. J. Appl. Phys. 31, L139 (1992). Therefore, the control of buffer layer growth is the most important step in the improvement of GaN main layer properties. The effect of buffer layer thickness and growth temperature on GaN main layer properties has been well studied: G. S. Sudhir, Y. Peyrot, J. Krüger, Y. Kim, R. Klockenbrink, C. Kisielowski, M. D. Rubin and E. R. Weber, Mat. Res. Symp. Proc. 482, pp. 525-530 (1998); Y. Kim, R. Klockenbrink, C. Kisielowski, J. Krüger, D. Corlatan, Sudhir G. S., Y. Peyrot, Y. Cho, M. Rubin, and E. R. Weber, Mat. Res. Symp. Proc. 482, pp. 217-222 (1998); J. Krüger, Sudhir G. S., D. Corlatan, Y. Cho, Y. Kim, R. Klockenbrink, S. Rouvimov, Z. Liliental-Weber, C. Kisielowski, M. Rubin and E. R. Weber, Mat. Res. Symp. Proc. 482 pp. 447-452 (1998). Buffer layers for Group-III nitride growth has been discussed in Mohammad et al., “Progress and Prospects of Group-III Nitride Semiconductors”, Prog. Quant. Electr. 1996, Vol. 20, No. 5/6 pp. 418-419, hereby incorporated by reference in its entirety. Various buffer materials are disclosed. Not disclosed or fairly suggested is a buffer layer of HfN.
Group III nitride semiconductors are discussed generally in Mohammad et al., “Progress and Prospects of Group-III Nitride Semiconductors”, Prog. Quant Electr. 1996, Vol. 20, No. 5/6 pp. 361-525, the contents of which are hereby incorporated in its entirety.
Other U.S. patents relevant to the state of the art include U.S. Pat. Nos. 5,369,289; 6,133,589; 5,767,581; 6,013,937; 5,578,839 and 5,290,393. U.S. Pat. No. 5,369,289 discloses a gallium nitride based compound semiconductor light emitting device comprising a buffer layer of a gallium nitride compound. U.S. Pat. No. 6,133,589 discloses an AlGaInN based light emitting diode having a buffer layer comprising a AlGaInN-based material. U.S. Pat. No. 5,767,581 discloses a gallium nitride based III-V group compound semiconductor having an ohmic electrode comprising a metallic material. U.S. Pat. No. 6,013,937 discloses a silicon wafer having a buffer layer formed on the dielectric layer. U.S. Pat. No. 5,578,839 discloses a gallium nitride based compound semiconductor device. U.S. Pat. No. 5,290,393 discloses a gallium nitride based compound semiconductor having a buffer layer of GaAlN. The above-mentioned references and U.S. patents are hereby incorporated by reference into this specification in their entirety.
Epitaxy of GaN on silicon offers a considerable cost advantage relative to growth on sapphire or SiC and the potential for monolithic integration of GaN-based devices with conventional microelectronics. However, Si substrates present additional challenges for GaN growth. Thick (>1 μm) GaN epilayers often crack upon cooling to room temperature due to the severe tensile stress induced by the ˜35% smaller thermal expansion coefficient of Si. Additionally, gallium exhibits poor wetting on the Si surface and exposed regions are converted to amorphous SiNx, disrupting epitaxy. Therefore low-temperature GaN (as typically employed on sapphire) is not an effective buffer layer for Si substrates and other materials must be considered. Buffer materials investigated include AlN, SiC, AlAs, intentionally formed SiNx, and BP. The best results by far have been achieved with the AlN buffer layer process, leading to the demonstration of high-brightness blue light-emitting diodes on Si. However, the mutual solubility of Si and Al is high at the buffer layer temperature (eutectic point 577C). Therefore AN may exacerbate interdiffusion at the interface, which results in high unintentional doping levels in both the film and substrate. These drawbacks merit further investigation of alternative buffer layers.